1. Field of Use
This invention relates generally to protective circuits for d.c. motors having armatures which are supplied with power from a.c./d.c. thyristor converters. In particular, it relates to protective circuits which are responsive to over-current faults to interrupt d.c. current flow from the converter to the motor armature and divert current flow generated by the motor armature until armature rotation can be stopped and the converter disconnected.
2. Description of the Prior Art
Heavy equipment or heavy machinery, such as large electric mining shovels or the like, employ extremely large electric motors (in the range of 100 horsepower on up) to drive machine components and impart hoist, swing and other motions thereto. Very often, these are d.c. motors whose armatures are supplied with d.c. power from three-phase a.c. power sources by means of a.c./d.c. thyristor converters with employ SCR's, thyratrons, or similarly functioning triggerable rectifier-type devices.
As a practical matter, any fault condition which gives rise to a short circuit across the motor armature causes armature current to increase rapidly and greatly, limited only by the resistance and inductance of the armature. As a result, flash-over or arcing occurs between the motor brushes and motor commutator which, if not extinguished quickly, causes a damaging ring to be burned into the commutator which is time-consuming and costly to repair.
Heretofore, it was the practice to employ protective devices, such a fast-acting mechanical d.c. circuit breakers, contactors or switches in the armature loop of large converter-supplied d.c. motors. These devices were responsive to a fault condition to interrupt current flow and extinguish the arc. But this prior art solution has three major drawbacks. The first major drawback is that such other devices are too slow to limit current and prevent motor flash-over. Second, since such protective devices need to be very large physically in order to quickly and effectively extinguish the arc, they are unsuitable for mounting on mobile equipment such as electric mining shovels. Third, since such protective devices are electromechanical in nature and subject to heavy physical shocks and vibrations during operation of a mining shovel, or other mobile equipment, they are unreliable in that they can operate accidently and cause nuisance shut-downs of the equipment.
U.S. Pat. No. 4,150,325 filed Dec. 2, 1977 and issued April 17, 1979 discloses forced firing of certain thyristors in a motor control system to achieve corrective action.
U.S. Pat. No. 3,584,282 filed April 1, 1969 and issued June 8, 1971 discloses use of a current limiting resistor in a motor armature circuit but employs a capacitor to modify or control thyristor firing time.
U.S. Pat. No. 4,074,175 filed April 15, 1976 and issued Feb. 14, 1978 shows an inductive load current measuring.
U.S. Pat. No. 4,151,450 filed July 28, 1977 and issued April 24, 1979 shows a circuit for protecting a d.c. motor against excessive current.
U.S. Pat. No. 3,590,350 filed Aug. 30, 1968 and issued June 29, 1971 shows a motor control for a hoist drive.